Calculation of B and O NMR parameters in molecular models for B 2O 3 and alkali borate glasses

Abstract

To help in the interpretation of the 11B and 17O magic-angle-spinning (MAS) and double-rotation nuclear magnetic resonance (DOR NMR) spectra of B 2O 3 and related glasses we have calculated NMR parameters from ab initio self-consistent-field MO theory for molecules modeling the local B and O environments of the glasses. The present work employs larger model molecules, larger basis sets, and a better quantum mechanical method (the gauge-including atomic orbital version of coupled Hartree-Fock perturbation theory) than in my previous studies. The calculated 11B shieldings confirm the deshielding of the B atoms within boroxol rings, B 3O 3(-O) 3, compared to corner sharing BO 3 units. The shielding of B atoms within the boroxol rings is almost unchanged when two or even three boroxol rings are linked together, but the shielding of B atoms in the corner-sharing BO 3 units is quite sensitive to the size of the cluster. Calculated O NMR shieldings and quadrupole coupling constants indicate that O in boroxol rings is deshielded and has a smaller coupling constant than O bridging between triangular BO 3 units. Calculated 17O shieldings are smaller for atoms in the middle of a cluster than for chemically similar atoms on the periphery. O atoms bridging between two boroxol rings or between a boroxol ring and a BO 3 unit have intermediate values for both shielding and coupling constant. 17O shieldings show a considerable range and seem more sensitive to distant neighbors than do the 11B shieldings. Perturbations of the boroxol ring model by deprotonation, or by interaction with Na + or OH −, change the 11B shielding by only 1 ppm, while the 17O shieldings and the frequencies of the Raman active symmetric ring O breathing modes show substantial changes. Thus, boroxol ring-like 11B NMR signatures may remain while boroxol-ring Raman spectral signatures disappear.